U.K. legal advice confirms London can’t unilaterally leave Brexit backstop

British lawmakers opened Round Two in their bruising battle with Prime Minister Theresa May’s government over her Brexit deal, as the release of previously secret legal advice provided more ammunition for opponents of her unpopular divorce agreement with the European Union.

May’s Conservative government is struggling to convince skeptical legislators that the divorce deal it has struck with the EU is a good one ahead of a vote in Parliament on Tuesday that could sink the agreement and possibly cost May her job.

May’s deal is in trouble because politicians on both sides of Britain’s EU membership debate oppose it. Pro-Brexit lawmakers say it keeps Britain bound too closely to the EU; pro-EU politicians say it erects barriers between the U.K. and its biggest trading partner and leaves many details of the future relationship undecided, subject to negotiations after Britain leaves the bloc on March 29.

A defeat in the vote next week would leave the U.K. facing a messy, economically damaging “no-deal” Brexit on March 29 and could topple the prime minister, her government, or both. EU officials have said they agreed upon a Brexit deal with May, and there’s no “Plan B.”

Opening the second of five days of debate, Home Secretary Sajid Javid told legislators Wednesday they should back the Brexit agreement to safeguard Britain’s vital security relationship with the EU.

“No one can pretend that this deal is perfect in every sense,” Javid acknowledged. But he said the alternative was “an uncooperative no-deal” Brexit that would shut Britain out of EU security tools and EU data-sharing organizations.

“It is my belief that the deal on the table is the best option available in ensuring a smooth exit from the European Union,” Javid said.

But opposition Labour Party lawmaker Chris Bryant said the agreement’s vision for future relations with the EU was “no more deliverable than a letter to Santa Claus.”

“How could a serious member of Parliament vote for nothing more than a wish list?” he said.

May is struggling to keep the Brexit deal on track after her government was dealt a double blow by Parliament.

In a historic first, legislators on Tuesday found the government in contempt of Parliament for refusing to publish legal advice it received from the country’s top law officer about the Brexit agreement. The government had argued that such advice is customarily kept secret. But it bowed to defeat Wednesday and released the reasoning from Attorney General Geoffrey Cox.

The main thrust of Cox’s advice was already known — the government released a 43-page document about it Monday in a bid to fend off the contempt motion. But the defeat demonstrated the fragility of May’s government, which does not have a majority in Parliament.

The legal advice also provided fuel for May’s opponents, who dislike a “backstop” provision in the Brexit agreement that would keep Britain in a customs union with the EU to guarantee an open border between EU member Ireland and the U.K.’s Northern Ireland.

The backstop is intended as a temporary measure, but pro-Brexit lawmakers say it could leave Britain tied to the EU indefinitely and unable to strike new trade deals around the world.

The legal advice confirmed that Britain can’t unilaterally opt out of the backstop, which requires an agreement by both sides. Cox advised there was a risk the U.K. might become stuck in “protracted and repeating rounds of negotiations.”

The border backstop is strongly opposed by Northern Ireland’s Democratic Unionist Party, which props up May’s minority government. The Protestant, pro-British DUP says the deal weakens the bonds of the United Kingdom by treating Northern Ireland differently than the rest of the country.

DUP deputy leader Nigel Dodds said the backstop “is totally unacceptable to unionists throughout the United Kingdom and it must be defeated.”

The DUP and pro-Brexit Conservatives want May to go back to the EU and try to renegotiate the backstop. Both May and the bloc insist the withdrawal agreement can’t be changed, and rejecting it means leaving without a Brexit deal.

In another blow to May, two dozen Conservative lawmakers voted with the opposition Tuesday to force an amendment to Brexit plans that gives lawmakers more say over what happens next if the deal is defeated in Parliament.

Pro-EU legislators say the amendment makes the prospect of a “no-deal” Brexit less likely, because Parliament can direct the government to take that option off the table.

Brexit-supporting legislators worry that opponents of Brexit in Parliament may try to water down the terms of departure from the EU, or even reverse the country’s decision to leave.

International Trade Secretary Liam Fox said Wednesday there was “a real danger that the House of Commons, which has a natural ‘Remain’ majority, may attempt to steal Brexit from the British people.”

Fox said that would be “a democratic affront” to the 52 percent of British voters who opted in a 2016 referendum to leave the bloc.